Pearson: Fishes of the Rio Beni Basin 
17 
bands replacing the spots. It would be difficult to divide these 
into varieties for in each group there are some which run into 
the other groups. 
Pygidium barbouri Eigenmann. 
17100, 31, 54-155 mm., Popoi River, Upper Beni. Sept., 1921. 
17101, 34, 25-105 mm., Espia. Aug., 1921. 
17102, 52, 30-120 mm., Rio Colorado, Lower Bopi. Sept., 1921. 
Comparison of the smaller specimens with a cotype shows 
them to be the same. The cotype has a less deeply emarginate 
caudal than the figure shows (Eigenmann, “The Pygidiidse.” 
Mem. Carnegie Mus., VII. No. 5, page 303, fig. 9) . 
Head in a specimen of 54 mm. 5.2, in a specimen of 155 mm. 
6.2 ; depth 6 to 7.2 ; D.13 ; A. 10 to 11 ; eye less than 3 in the 
interocular, 8 to 9 in the head, slightly nearer the snout than 
the posterior part of the head ; tail strongly compressed ; head 
about 1.2 longer than wide; nasal barbels reaching to the pos- 
terior margin of the head or slightly beyond ; the longer maxil- 
lary barbels extending somewhat beyond the origin of the 
pectorals ; teeth conical ; first pectoral ray prolonged, the ray 
and prolongation slightly greater than the head; dorsal sub- 
truncate, none of its rays prolonged ; the distance of the origin 
of the dorsal from the caudal 2.6 to 3 in the length ; origin of 
the anal under the tenth or eleventh dorsal ray; caudal trun- 
cate; accessory caudal rays in some conspicuous in others not 
conspicuous ; ventrals extending to anal, in the young slightly 
beyond. A dark median band which extends thru the caudal, 
in the larger specimens indistinct; a less distinct line above 
the median line; general color from light brown in young to 
dark brown in the old. 
Pygidium eichorniarum (Ribeiro). 
17103, 2, 45 and 50 mm., Lagoons, Lake Rogoagua. Nov., 1921. 
Urinophilus erythrurus Eigenmann. 
17104, 1, 75 mm., Rurrenabaque. Nov., 1921. 
Tridens brevis Eigenmann & Eigenmann. 
17277, 20, 22-26 mm., Lagoons, Lake Rogoagua, Bolivia. Nov., 1921. 
Heretofore T. brevis has been known only from the type in 
the Museum of Comparative Zoology, Cambridge, Mass. It 
has not been found in a search for it recently. 
